This invention relates to a system for controlling vehicle safety devices such as an air bag, a seat belt tightening device and the like.
Recently, in order to ensure the safety of motor vehicle passengers, various safety devices such as an air bag, a seat belt fastening device and the like have been . It must be assured for all of these safety devices that they are ignited at the time when a vehicle collision has occurred, and that they are not ignited accidentally when a vehicle is normally running.
As disclosed in U.S. Pat. No. 5,155,376, in a conventional air bag device, a squib operation circuit is provided between a battery which is mounted on a vehicle and the ground. This squib operation circuit comprises a transistor (main switching means), a squib (activator) for an air bag, and a safety switch, all arranged in series. When electric current is supplied to the squib, the latter is ignited to inflate or expand the air bag.
The above control system further comprises an acceleration sensor for detecting the acceleration of the vehicle, and a microcomputer for judging whether or not a vehicle collision has occurred in accordance with data from the acceleration sensor. When the microcomputer judges that the vehicle collision has occurred, it outputs an operation instruction signal to turn on the above transistor in order to provide electric current to the squib.
The safety switch is designed such that when the vehicle is in a normal running condition, the safety switch is held in an OFF-state so that electric current may not flow to the squib, and the safety switch is turned on in response to an impact with a lower level than that of an impact in response to which the microcomputer may judge that a vehicle collision has occurred. Owing to this arrangement, even if the operation instruction signal is outputted as a result of runaway of the microcomputer and the transistor is turned on when the vehicle is in the normal traveling condition, the air bag can be prevented from being accidentally expanded because the safety switch is kept in the OFF-state.
Also connected to the operation circuit for the squib is a condenser with a large capacity which can be charged by the battery. Since the squib operation circuit works by using the condenser as a power source even in the event where the harness between the squib operation circuit and the battery is cut off at the time when the vehicle collision has occurred, the air bag is assuredly made to expand.
The above condenser is also acts as a backup power source for the use of the microcomputer. As a consequence, even in the event where the harness of the battery is cut off as mentioned above, continued operation of the microcomputer is ensured.
It is desirable that the microcomputer limits the time period for the transistor to be kept in the ON-state (i.e., the time period for providing electric current to the squib) to the time period required for the ignition of the squib. The reasons are as follows. Presuming that the harness of the battery is cut off at the time the vehicle collision has occurred, if the time period for supplying electric current to the squib is too long, the electric load of the condenser is consumed by the supply of electric current to the squib. As a result, the condenser may become unable to act as the backup power source for the use of the microcomputer. The result is that the microcomputer may not operate any further, thus making it impossible for the microcomputer to write the record of controlling the air bag into a non-volatile memory, for example.
In the above control system, the safety switch is duly expected to be in the ON-state during a preset time period where the transistor is held in the ON-state. The reason is that the safety switch is turned on by an impact with a lower level than that of the impact in response to which the microcomputer judges that a vehicle collision has occurred. In this way, only when both the transistor and safety device are in the ON-states, electric current is supplied to the squib to expand the air bag. However, depending on the types or conditions of vehicle collision, there are instances where the safety device is repeatedly turned on and off due to chattering. Nevertheless the transistor is in the ON-state, and no electric current is supplied to the squib as long as the safety device is in the OFF-state. Accordingly, if the time period for holding the transistor in the ON-state is set too short without taking into consideration the time period where the safety switch is in the OFF-state, there is a possibility, in the worst case, that electric current is not supplied to the squib for a long time enough and as a result, the air bag is not expanded.
For this reason, actually, the time period for outputting the operation instruction signal is obliged to be set longer than the time period required for expanding the air bag taking into consideration the possible chattering of the safety switch. This attempt, however, brings about another problem in that if the chattering does not occur to the safety switch at the time when the vehicle collision has occurred, electric current is supplied to the squib for a longer time than that required for expanding the air bag. This results in a large consumption of electric power being accumulated in the condenser. As a consequence, it is necessary to make the capacity of the condenser large enough to store sufficient electric power write the record of the controlling of the air bag into the nonvolatile memory after the supply of electric current to the squib.
The related art, e.g., Japanese Early Laid-Open Patent Application No. 246139/91 (corresponding to U.S. patent application Ser. No. 07/651626, issued as U.S. Pat. No. 5,283,472) discloses a means for judging the condition of the safety switch. According to this art, if it is judged that the safety switch is in the ON-state at the same time when it is judged that an acceleration sensor is subjected to a failure, an operation instruction signal is outputted to a transistor and as a result, an air bag is expanded. Japanese Early Laid-Open Patent Application No. 79450/1991 also discloses a means for checking the condition of a safety switch. According to this prior art, when an actual condition of the safety switch is different from the condition of the safety switch which can be expected when the safety switch is in a normal condition, it is judged that the safety switch is subjected to a failure. U.S. Pat. No. 5,101,192 discloses that a microcomputer stores failure judgment data, etc. of an air bag system is stored in a non-volatile memory and electric power of a condenser is consumed for this storing operation.